The paradigm that inflammation stimulates adaptive immunity is well established. Paradoxically, mounting evidence reveals that some forms of inflammation create local suppression, particularly in tissues subjected to chronic inflammation caused by persistent infections and developing tumors. Suppressive inflammation may facilitate tumor progression and promote pathogen persistence in immunocompetent individuals by inhibiting T cell responses to tumor and pathogen antigens encountered in these tissues and in associated draining lymph nodes. However, the underlying mechanisms that create and maintain local suppression in inflamed tissues are not well defined. Elucidating the mechanisms that promote local suppression in inflamed tissues will provide critical new insights and opportunities to improve therapeutic manipulations to treat hyper-immune syndromes such as transplant rejection, and autoimmune and allergic diseases by introducing donor (all) and self (auto) antigens at the same time as boosting immune suppressor activity. In previous work, we identified rare populations of plasmacytoid dendritic cells (pDCs) in humans and mice uniquely competent to express the immunosuppressive enzyme indoleamine 2,3 dioxygenase (IDO). In mice, interferon type I (IFN1) was the obligate upstream IDO inducer in splenic IDO-competent pDCs after B7 ligation. However, IFN1 production by pDCs was dependent on IDO activity, revealing that IDO amplified its own expression in IDO-competent pDCs via a cell autonomous IFN1-dependent pathway. Unlike their counterparts in spleen, pDCs in draining lymph nodes (dLNs) associated with melanoma growth expressed IDO constitutively, exhibited potent suppressor activity and activated regulatory CD4+CD25+ T cells (Tregs). We hypothesize that the ability of IDO-competent pDCs to create local suppression is critically dependent on cooperative interactions between pDCs and Tregs that amplify IDO expression, which triggers suppressor activity in both cell types. In preliminary studies we show that topical treatment of mice with the pro-inflammatory reagent and tumor promoter phorbol myristate acetate (PMA) paradoxically creates potent local suppression in skin dLNs by activating pDCs to express IDO, which in turn activate Tregs via IDO. The objective of studies proposed is to elucidate the mechanisms that induce dLN pDCs to express IDO (Aim 1), and activate Treg bystander suppressor activity via IDO (Aim 2) after PMA treatment. New knowledge generated will be used to guide and evaluate innovative methods to enhance local IDO expression during immunization, skin grafting and during autoimmune type I diabetes progression to block T cell mediated destruction of skin allografts and pancreatic islet cells, respectively. PUBLIC HEALTH RELEVANCE Studies proposed focus on a small subset of dendritic cells that completely suppress T cell responses to antigens in inflamed tissues when they receive particular signals from the tissue microenvironment. To create suppressive tissue microenvironments, dendritic cells must interact with a small subset of regulatory T cells via specific signaling pathways. Recently, we discovered that skin inflammation provoked by `painting'with a pro-inflammatory reagent commonly used in mouse models of tumor progression induces potent suppressor activity mediated by dendritic cells in skin draining lymph nodes. This observation provides a rationale for treating mice with donor allograft and self-antigens to induce tolerance that will protect skin grafts and pancreatic islet cells from T cell mediated destruction. 1